The increasing demand of lighter products and middle distillates will continue in the near future and at same time the demand of heavy crudes is decreasing. To meet this demand a substantial quantity of heavy crudes has to be upgraded. Since the heavier crudes contain various types of contaminants the refining processes are modified technologically. The technological change can not only handle with very tighter environmental legislation. Therefore, several efforts are made for making new generation of catalysts which are more active, more selective, more life time and more thermally stable.
The presence of high percentage of asphaltene is the main concern during hydroprocessing since these compounds are the primary cause of catalysts deactivation. It is the main precursor of coke. It is the main precursor for formation of coke. Deactivation of hydroprocessing catalysts is found on time-on-stream. Activity changes rapidly during the first few hours of run and then it becomes stable. Coke equivalent to up to 25 wt % of the original catalyst is deposited within few hours of run and this can decrease surface area up to 50 to 60% of the original catalyst. Loss of surface area is occurred due to blockage of small pore of the catalyst.
The presence of vanadium and nickel is of particular importance because of their poisoning effect during hydrodesulfurization and cracking of the feeds. The metals are usually distributed between porphyrin and nonpophyrin type of structures. These metal containing compounds are deposited into the catalyst during hydrotreating. Because of their large size they do not penetrate deeply into the catalyst. They are accumulated as metal sulfides into the pore mouth of the catalyst and block the way to enter the reactants. This is cause for deactivation of catalyst. Therefore, the main features of a HDM catalyst are very large pore diameter and large pore volume. It has to be very high metal storage capacity.
To develop new type of hydroprocessing catalysts various kind of approaches are have been reported such as the use of different supports materials, different active metals, modified support and catalyst by using several additives, etc.
The use of carbon support for hydrotreating catalyst is also investigated. It was also found that sometimes, carbon supported catalyst shows higher HDS activity compared with catalysts supported by conventional carrier like Al2O3 and SiO2. The use of carbon support has several advantages. It has very high surface area, so higher metals impregnation can be done. It is easy to control pore structure so that diffusional limitation can be minimized. The recovery of the active metals or deposited metals during reaction is easy; just it can be done by burning off carbon carrier. This carbon support is particularly attractive for hydrotreating of heavy crude oils or residue because it reduces coke deposition tendency, which as it has been discussed earlier, is the main concern for HDT of heavy crude oil and residue. It causes rapid deactivation of the catalyst. One of the major disadvantages of the use of this carbon support is its mechanical resistance. However, mechanical strength can be improved by using different kind of binding materials.
The raw carbon material is in general inert and this material can not be used as such for carrier to prepare HDT catalyst. There are several methods to improve its basic and acid characteristics such as steam treatment, HNO3, Na2CO3, NaOH, HCl oxidation. It was reported that raw C has very small basic sites and 10 times higher content of very week acid sites. When this raw C is treated with steam the basic sites are generated in expense of week acid sites. However, when it is activated with nitric acid or (NH4)2S2O8, it develops acidic sites at the cost of basic sites. During oxidation of C with (NH4)2S2O8 it produces carboxylic groups on the carrier surface which are highly acidic in strength.
The support-metal interaction of carbon carrier is comparatively week. Therefore, sulfidation of carbon supported catalyst is more effective and hence one can expect more active sites on this type of catalyst. Due to its week interaction the formation of CoMoS type II structure is more favorable on this catalyst. In general, CoMoS type I is formed on the alumina supported catalyst at lower sulfidation temperature. When temperature raises the formation of type II CoMoS structure is predominant which is comparatively high active sites than type I. Mössbauer emission and X-ray absorption spectroscopy studies on sulfided catalyst have shown that the presence of CoMoS may be abundant on such a catalyst. Other studies also showed that the formation of more active octahedral species is easy on carbon supported catalysts. The thiophene HDS activity of carbon supported catalyst is very high and also depends on the acid characteristics of the surface. It is found that raw carbon treated with (NH4)2S2O8 is more acidic than that of treated with HNO3. It is manly due to the formation of carboxylic group on the carbon carrier when it is treated with (NH4)2S2O8. These acidic sites enhance the thiophene HDS activity. However, weak interaction of active metal and carbon has high probability of the formation of bulk metal sulfide during activation of the catalyst and it causes decrease of HDS activity. It is also proposed that the active metals and sulfur compound of the reactant is strong on the carbon supported catalyst resulting higher activity.
There are several approaches to development suitable hydrotreating catalysts for heavy crude oils and residue. One of them is the improvement of pore structure of the carrier materials used to prepare the catalyst. An enlarging of pore diameters and higher pore volume are the most suitable options to minimize diffusional restriction and improve metals retention capacity of the catalyst. Various methods were employed for the preparation of catalysts for hydrotreating of heavy feedstocks and for the processes for the same in the following U.S. Pat. Nos. 3,770,617; 3,864,416; 4,016,106, 4,082,695; 4,328,127; 4,332,782; 4,422,960; 4,456,701; 4,508,841; 4,520,128; 4,572,778; 4,613,427; 4,870,044; 5,210,061; 5,531,976; 5,928,499; 6,015,485.
U.S. Pat. No. 5,472,595 is directed to the use of carbon support for hydrodesulfurization and hydrodearomatization of a light atmospheric gas oil. The catalyst supported on carbon having 600 m2/g specific surface area, about 0.3 cc/g total pore volume, at least 12 Å average pore diameter is loaded by about 35.9 wt % W, 7.0 wt % Ni and 1.1 wt % P. The sulfided catalyst is tested with light gas oil in presence of hydrogen at 340° C. temperature, 53 kg/cm2 pressure, 2.0 h−1 LHSV and 356 m3/m3 hydrogen flow rate.
U.S. Pat. No. 5,837,640 disclosed the use of carbon support for preparation of hydrodearomatization and hydrodesulfurization catalysts. The catalyst having BET specific surface area of 900 m2/g, pore volume of 0.8 cc/g, pore diameter of 20 Å is loaded by about 18.6 wt % of MoO3, 3.8 wt % of NiO. Ammonium heptamolybdate and nickel nitrate are used for metal and promoter loading respectively. The catalyst is tested with middle distillate in presence of hydrogen at temperature of 380° C., pressure of 100 kg/cm2, LHSV of 1 h−1 and hydrogen flow of 713 m3/m3.
U.S. Pat. No. 6,162,351 discloses the use of carbon support to prepare hydrodenitrogenation catalyst. The catalyst is employed for removal of aromatic and nitrogen compounds from middle distillate. The catalyst has high specific surface area carbon support of 1600 m2/g, nitrogen pore volume of 0.82 cc/g and an average pore diameter of 20 Å. The catalyst contains about 12 wt % of molybdenum, 5 wt % of cobalt and 3 wt % of chromium.
These prior art works reveal a continuous change of hydrotreating processes and an improvement of catalytic activity. However, the major problem in the present scenario is the increasing production of heavy crude oils and decreasing of the demand of bottom products. It needs the upgradation of heavy crude oils and residue.
All references discussed above provide catalysts which are usually deactivated rapidly; this invention provides a catalyst supported on activated carbon, which maintains a high stability for the hydroprocessing of heavy crude oil and residues.